In a gas turbine engine and, in particular, an aircraft gas turbine engine, compressor stalls must be avoided as much as possible. There are different types of stalls. One, a full span non-recoverable rotating or rotary stall, brings about a major loss in thrust because a large section of the compressor stops pumping and the compressor does not return to normal operation. Less serious compressor stalls appear as partial span rotating stalls near the blade tips. However, these partial span rotating stalls can lead to the undesirable compressor "surge" phenomenon in which the complete flow transiently reverses followed by a recovery to nearly normal operation. The flow reversal and subsequent recovery "attempt" can repeat for several cycles. A compressor with good recoverability characteristics will automatically return to normal operation after very few surge cycles. The momentary thrust loss for a portion of a second during surge and recovery can have minimal impact on aircraft performance; however, surge must be avoided since it can cause structural damage. Also surge can sometimes transition into the highly dangerous mode of non-recoverable full span rotating stall, which results in very low thrust as well as turbine damage, from overheating.
Companion patent applications titled "On-Line Stall Margin Adjustment in a Gas Turbine Engine" and "Controlling Stall Margin in a Gas Turbine Engine During Acceleration", Ser. No. 07/789,556 and 07/789,557 (U.S. Pat. No. 5,165,845) respectively, both filed on Nov. 8, 1991, and assigned to the assignee hereof, concern techniques for preventing partial span stalls and surges. Full span rotating stalls on the other hand, are associated with a deficiency in the engine system's recoverability characteristics. In that regard, investigations have shown that engine recoverability is dependent on many factors including compressor design parameters, engine components, volumes, combustion stability and engine cycle.
Mass flow through the compressor stage is a function of several factors. In a full-span rotating stall, mass flow can be so small that the resulting high fuel air ratios cause internal engine temperatures to rise to potentially destructive levels. If allowed to continue, this "hung" stall condition can lead to serious engine damage. When the compressor in it is a hung stall, the pilot must shut down the engine and then try to restart it, something that is not easy to do especially quickly.